1. Field of the Invention
This invention relates to a plastic scintillator. To be more particular, this invention relates to a plastic scintillator which possesses improved physical properties.
2. Description of Prior Arts
When radiations comprising electrically charged particles such as .alpha.-rays and .beta.-rays penetrate a certain substance, they ionize, excite or dissociate atoms or molecules of the substance at the cost of their energy. On the other hand, the energy thus lost by the radiation and accumulated in the substance is either converted into energy of the form of thermal movement or just emitted in the form of electromagnetic waves. Particularly when the substance so penetrated by the radiation is fluorescent or phosphorescent, a fair portion of the excited energy is emitted in the form of light of a wavelength in the visible zone. This phenomenon of light emission is called "scintillation". Also in the case of radiation comprising .gamma.-rays and neutron rays which are devoid of electric charge, a similar phenomenon is induced by the action of secondary charged particles which are emitted when the radiation interact with a substance. Generally, therefore, this phenomenon is widely utilized for the detection of radiation.
Substances capable of causing the scintillation are generally called scintillators. Examples of scintillators are inorganic crystals represented by sodium iodide activated with thallium, organic crystals represented by anthracene, organic solutions represented by xylene solution of terphenyl and plastic scintillators represented by terphenyl-polystyrene. These substances are extensively used as luminous bodies for the detection of radiation. Among other scintillators, particularly plastic scintillators are easy to handle and are readily moldable in desired, large shapes and, owing to these merits, have come to find utility as indispensable devices in the field of researches on cosmic rays and researches on high-energy physics by use of particle accelerators. In recent years in the field of researches on high-energy physics, development of large particle accelerators has increased demand for a great quantity of large plastic scintillators. In the properties which are expected from such plastic scintillators, high processibility has now become an important requirement besides those basic properties of scintillators in general which are represented by amount of emission and transparency.
The transparent resins heretofore used in plastic scintillators have been limited to styrene-based resins such as, for example, polystyrene and polyvinyl toluene. The plastic scintillators using these resins as their matrix resin have suffered from the disadvantage that the resins themselves are expensive and they are deficient in processibility. For example, such a plastic scintillator more often than not utilizes the whole reflection of light on the inner surfaces thereof to permit effective transfer of the light generated therein to a detector such as a photomultiplier tube. The plastic scintillator, therefore, is generally used in a highly polished state. The plastic scintillator which uses the conventional styrene-based resin, however, is liable to sustain cracks when it is polished. When the scintillator is wiped with a solvent such as alcohol to have its surface cleaned, it tends to sustain cracks while in use. Because of such defective physical properties as described above, the conventional styrene-based plastic scintillators, despite the outstanding merit of high emission efficiency, call for advanced skill and great toil on the part of users engaging in the work of polishing and handling these plastic scintillators. In applications which demand heavy consumption of large plastic scintillators, therefore, the conventional styrene-based plastic scintillators have not necessarily proved quite practical.
To overcome the drawbacks mentioned above, plastic scintillators using inexpensive acrylic resin as matrix resin have been developed in recent years. Unlike styrene-based resins acrylic resin itself is not a scintillator. These plastic scintillators, therefore, have a fatal disadvantage that they are deficient in the emission efficiency which constitutes the basic property for plastic scintillators. This particular disadvantage is corrected to some extent by having a scintillating substance such as, for example, naphthalene or styrene dissolved in a high concentration in the acrylic resin [Nuclear Instruments and Methods 169, 57-64(1980)]. Since the improved scintillators contain the scintillating substance in a high concentration, they are inevitably deprived of the outstanding mechanical properties inherent in acrylic resin. Generally as the scintillating substance to be contained in a high concentration in the acrylic resin, naphthalene is used in an amount of 1 to 15% by weight in due consideration of solubility and cost. Plastic scintillators incorporating naphthalene have a disadvantage that the time for the attenuation of light which ensues from the penetration of the scintillators by radiation is long, i.e. the response of the scintillators to the radiation is slow. These drawbacks restrict the range of applications found for the plastic scintillators using acrylic resin as their matrix resin.
An object of this invention, therefore, is to provide a novel plastic scintillator. Another object of this invention is to provide a novel plastic scintillator which favorably compares with the conventional styrene-based plastic scintillators in the basic properties of plastic scintillators represented by emission efficiency, transparency and response time and possesses outstanding practical properties.